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Lemna perpusilla and Lemna minor are free‐floating plants that often live in the same habitat. However, little is known about how they differ in response to herbivore attacks. In this study, we examined the species‐specific resistance of two Lemna species to the duckweed weevil, Tanysphyrus lemnae. The female adults of T. lemnae preferred to lay eggs on L. perpusilla over L. minor. In addition, the larvae of T. lemnae performed better when fed on L. perpusilla than on L. minor. To understand the physiological basis of species‐specific resistance in the two Lemna species, we measured the amounts of jasmonic acid (JA), phytosterols, and flavonoids. Attacks by duckweed weevils increased the levels of JA in the two Lemna species, but these levels did not differ significantly between the two species. Interestingly, the levels of flavones (isoorientin, vitexin, and isovitexin) in L. minor species were higher than those in L. perpusilla. The in vitro bioassay showed that three flavones significantly decreased the survival rate of duckweed weevil larvae. Although L. perpusilla was less resistant to duckweed weevil attack compared to L. minor, L. perpusilla grew faster than L. minor regardless of the duckweed weevil attack. These results suggest that these two Lemna species have different defense strategies against the duckweed weevil. [ABSTRACT FROM AUTHOR]

Herbivorous insects use plant metabolites to inform their host plant selection for oviposition. These host-selection behaviours are often consistent with the preference-performance hypothesis; females oviposit on hosts that maximize the performance of their offspring. However, the metabolites used for these oviposition choices and those responsible for differences in offspring performance remain unknown for ecologically relevant interactions. Here, we examined the host-selection behaviours of two sympatric weevils, the Datura ( Trichobaris compacta) and tobacco ( T. mucorea) weevils in field and glasshouse experiments with transgenic host plants specifically altered in different components of their secondary metabolism. Adult females of both species strongly preferred to feed on D. wrightii rather than on N. attenuata leaves, but T. mucorea preferred to oviposit on N. attenuata, while T. compacta oviposited only on D. wrightii. These oviposition behaviours increased offspring performance: T. compacta larvae only survived in D. wrightii stems and T. mucorea larvae survived better in N. attenuata than in D. wrightii stems. Choice assays with nicotine-free, JA-impaired, and sesquiterpene-over-produced isogenic N. attenuata plants revealed that although half of the T. compacta larvae survived in nicotine-free N. attenuata lines, nicotine did not influence the oviposition behaviours of both the nicotine-adapted and nicotine-sensitive species. JA-induced sesquiterpene volatiles are key compounds influencing T. mucorea females' oviposition choices, but these sesquiterpenes had no effect on larval performance. We conclude that adult females are able to choose the best host plant for their offspring and use chemicals different from those that influence larval performance to inform their oviposition decisions. [ABSTRACT FROM AUTHOR]